The invention relates to liquid crystal display devices, and more particularly to backlighting of such liquid crystal display devices.
The liquid crystal display, more commonly known as LCD, is well known to those of ordinary skill in the art. As briefly described in U.S. Pat. No. 5,046,829, Liquid Crystal Display Backlighting Apparatus Having Combined Optical Diffuser And Shock Dampening Properties, issued to Worp on Sep. 10, 1991, the complete disclosure of which is incorporated herein by reference, the LCD is made up of cells containing a thin layer of liquid crystal material between a plurality of segmentary electrodes. An electric field applied between the electrodes of any one of the segmentary electrodes causes the liquid crystal material therebetween to align with the electric field, and the light that passes through the polarizer on one side of the liquid crystal material is absorbed by the polarizer on the opposing side of the material, such that the absorbing polarizer appears xe2x80x9con.xe2x80x9d
Some LCD devices absorb or reflect ambient light impinging on the display face from outside the device. Therefore, ambient sun light or room light is required to view the display. These passive LCD devices have advantages of low power consumption combined with low weight and cost. Generally, however, passive LCD devices are inefficient in low ambient light conditions or at night. Therefore, LCD devices intended for low light conditions use internally supplied supplemental illumination. In a simple supplemental illumination system, one or more light sources, usually incandescent lamps, are placed behind the display, i.e., backlighting the display. One disadvantage of these simple supplemental illumination systems is lack of uniformity in the illumination. Incandescent lamps create localized xe2x80x9chot spotsxe2x80x9d which reduce the display""s readability. While an optical diffusion panel placed between the illumination source and the display more evenly distributes the light from the internal sources and helps correct xe2x80x9chot spots,xe2x80x9d unevenness in brightness is unavoidable absent some natural diffusion. Natural diffusion requires sufficient space between the illumination source and the display. However, thinness is a requirement of automobile dashboard or aircraft control panel displays which does not permit sufficient space for natural diffusion. One solution presented in U.S. Pat. No. 4,649,381, Liquid Crystal Panel Display Device, issued to Masuda et al. on Mar. 10, 1987, the complete disclosure of which is incorporated herein by reference, divides the display panel into xe2x80x9cdisplay blocks,xe2x80x9d each displaying information in different small sectors of the panel, each sector including its own dedicated illumination source behind the respective panel sector and an optical diffuser shared by all of the panel sectors. While dividing the panel into discrete sectors may result in sufficiently uniform illumination within each sector, such discrete sectors do not address the problem presented by a full screen graphical or textual display. Another disadvantage of the divided display is the complete loss of information in any discrete sector if the illumination source fails in that sector of the display.
Another disadvantage of incandescent lamps is the high energy costs of powering the lamps. Today, many LCD devices use florescent lamps to control energy costs. However, florescent lamps are subject to the same unevenness in illumination from which incandescent lamps suffer. Also, florescent lamps present additional drawbacks. For example, more sophisticated LCD devices include the ability to adapt the display""s light level to the ambient conditions. An automobile dashboard or aircraft control panel display usually includes a dimmer switch for adjusting the display brightness to a comfortable viewing level. Florescent lamps, however, require complex and expensive circuitry to adjust, or dim, the brightness of the supplemental internal illumination.
Another supplemental illumination configuration is side, or peripheral, lighting, which provides illumination at the sides of the display and uses a light guide to illuminate the interior portions of the display. Clearly, side lighting results in an unacceptably dark or under-lighted area in the central portion of a large area display due, at least in part, to attenuation at the illumination panel. In particular, in a large area LCD display, the central portion of the display remote from the illumination source is inevitably darker than the peripheral areas adjacent to the illumination source. To date, light guides in various configurations have attempted to adequately distribute the illumination rays across the display surface, as disclosed, for example, by U.S. Pat. No. 4,714,983, Uniform Emission Backlight, issued to Lang on Dec. 22, 1987; U.S. Pat. No. 4,929,062, Light Guide For LCD, issued to Guzik et al. on May 29, 1990; U.S. Pat. No. 4,729,185, Display Panel Illumination Device, issued to Baba on Mar. 8, 1988, the complete disclosures of which are incorporated herein by reference; and above incorporated U.S. Pat. No. 5,046,829. Such light guides are generally unsuccessful in providing uniform illumination of the display.
More recently, supplemental illumination has been provided using light emitting diodes, or LEDs. While the useful life or mean-time-between-failures (MTBF), is estimated at ten times (10xc3x97) or more of florescent lamps, the current high cost of LEDs has restricted their use to configurations using side lighting in combination with light guides, as disclosed for example by above incorporated U.S. Pat. No. 5,046,829. These attempts suffer the same limitation as light guides used with incandescent and florescent lamps: side lighting results in an unacceptably dark area in the central portion of a large area display remote from the illumination source.
Furthermore, side lighting of LCD displays with white light was previously possible using a mixture differently colored LEDs, i.e., using a mixture of LEDs radiating the three primary colors, the combination of which appears as white light to the viewer. However, LEDs radiating at some wavelengths are more expensive than those radiating at others due to differences in the chemical ingredients and manufacturing processes required to develop florescence at the proper wavelength. Therefore, the cost savings normally available from mounting a large number of same colored LEDs in a circuit has been unavailable in a white lighted LCD display.
Thus, until now, the long-felt need for a practical low cost, thin or low profile, backlighted large display LCD device having long lamp life, a high degree of illumination uniformity and simple brightness adjustment circuitry has been unattainable due to the nonuniformity of illumination using either florescent or incandescent lamps to illuminate large displays, even when combined with optical diffusers or light guides; the complex circuitry required to adjust brightness using florescent lamps; and the relatively high cost of colored LEDs. Furthermore, the long-felt need for such a practical low cost, thin or low profile, backlighted large display LCD device having white colored supplemental illumination sources has been unattainable due to the need to mix various differently colored LEDs to provide white side or back lighting.
The present invention overcomes display nonuniformity, display brightness control circuitry complexity, high cost, and short lamp life limitations of the prior art by providing a liquid crystal display device having essentially uniform backlighting in a flat panel display provided by multiple light emitting diodes mounted in backlighting relationship with a conventional liquid crystal display panel. The light emitting diodes providing a mean-time-between-failures, or MTBF, estimated at ten times (10xc3x97) that of prior florescent lamps. Furthermore, the illumination level or brightness of the light emitting diodes provided by the present invention is controlled by a relatively simple and less expensive circuit as compared with the circuit necessary for controlling the illumination level of the prior lamps.
According to one aspect of the present invention, the backlighted liquid crystal display device includes an optical chamber having two opposing spaced apart faces with a liquid crystal display panel mounted on one of the two faces and a printed circuit board mounted to the other of the two faces. Multiple light emitting diodes are arranged in a two-dimensional spatially uniform row and column array across essentially the entirety of the surface of the printed circuit board which is mounted to the optical chamber. The light emitting diode array thus mounted backlights the liquid crystal display. Preferably, the light emitting diodes radiate white light. Either each of the light emitting diodes radiates white colored light or the light emitting diodes are a mixture of light sources radiating in different ones of the three primary color bands, such that the combination of light emitting diodes generates white light.
According to another aspect of the present invention, the backlighted liquid crystal display device includes a brightness adjusting circuit electrically coupled to the light emitting diodes, the brightness adjusting circuit being relatively simple as compared with the complexity of a brightness adjusting circuit designed to control the light level of the florescent lamps of prior devices.
According to another aspect of the present invention, the backlighted liquid crystal display device includes a an optical diffusion panel mounted adjacent the light emitting diodes opposite the printed circuit board, i.e., between the light sources and the liquid crystal display panel. The optical diffusion panel forms a layered illumination assembly with the printed circuit board and light emitting diodes.
According to still another aspect of the present invention, the backlighted liquid crystal display device includes an optional heat sink mounted adjacent said printed circuit board opposite said light emitting diodes. Alternatively, the cooling function of the heat sink is performed by an optional fan mounted on or near the backlighted liquid crystal display device.
According to another aspect of the present invention, the surface of the printed circuit board having the light emitting diodes arranged thereon further includes a light-reflective finish formed thereon. Preferably, the internal surfaces of the optical chamber are also formed with a light-reflective finish.
According to still another aspect of the present invention, the invention provides a layered illumination assembly for mounting in a backlighting relationship with a liquid crystal display panel in a liquid crystal display device. Accordingly, the invention provides a two-dimensional arrangement of light emitting diodes mounted on one surface of a printed circuit board, the arrangement of light emitting diodes essentially filling the entirety of the surface; an optical diffusion panel mounted adjacent to the light emitting diodes opposite the printed circuit board; and an optional heat sink mounted adjacent the surface of the printed circuit board opposite the optical diffusion panel.
According to yet another aspect of the present invention, the invention provides a method of backlighting a liquid crystal display panel with white light using multiple light emitting diodes by mounting a liquid crystal display panel on one surface of an optical chamber; mounting the multiple light emitting diodes in a two-dimensional array essentially covering the surface of a printed circuit board, the light emitting diodes generating an essentially white colored light; mounting the printed circuit board on a surface of the optical chamber in a backlighting relationship to and spaced away from the liquid crystal display panel; and backlighting the liquid crystal display panel with an essentially uniform illumination.
According to various other aspects of the present invention, the method of the invention also provides a heat dissipating device mounted in layered relationship with the printed circuit board opposite to the light emitting diodes; an optical diffusing device mounted in layered relationship with the printed circuit board and the heat dissipating device opposite the heat dissipating device; and a brightness adjusting circuit coupled to the light emitting diodes.